Xianyun Ma

Inhomogeneities in Ni/4H-SiC Schottky barriers: Localized Fermi-level pinning by defect states

We investigated arrays of Ni, Pt, or Ti Schottky diodes on n-type 4H-SiC epitaxial layers using current-voltage (I-V) measurements, electron beam induced current (EBIC), polarized light microscopy, x-ray topography, and depth-resolved cathodoluminescence spectroscopy. A significant percentage of diodes (∼7%–30% depending on epitaxial growth method and diode size) displayed “nonideal” or inhomogeneous barrier height characteristics. We used a thermionic emission model based on two parallel diodes to determine the barrier heights and ideality factors of high- and low-barrier regions within individual nonideal diodes. Whereas high-barrier barrier heights increased with metal work function, low-barrier barrier heights remained constant at ∼0.60, 0.85, and 1.05eV. The sources of these nonidealities were investigated with a variety of spectroscopic and imaging techniques to determine the nature and energy levels of the defects. EBIC indicated that clusters of defects occurred in all inhomogeneous diodes. Cathod...

Nondestructive defect delineation in SiC wafers based on an optical stress technique

The potential of using the optical stress technique to delineate the various defects in silicon carbide (SiC) wafers has been fully demonstrated. The observed defects include micropipes, dislocations, stress striations, grain boundary or dislocation walls, and regions of polytype nonuniformity. Revealed dislocation density is in the range 104–105 cm−2.

Extended SiC Defects: Polarized Light Microscopy Delineation and Synchrotron White-Beam X-Ray Topography Ratification

A simple imaging technique using polarized light microscopy (PLM) has been developed to delineate and map the defects in silicon carbide (SiC) wafers. The correlation of different defects using synchrotron white-beam X-ray topography (SWBXT) and PLM has been established. The previously reported wave-shaped PLM features observed using PLM are confirmed to be screw dislocations by SWBXT. PLM can be used for rapid assessment of the quality of SiC substrates and substrates with an epitaxial film.

Prebreakdown and breakdown investigation of broad area electrodes in the micrometric regime

The dc prebreakdown and breakdown characteristics of micrometric gaps varying from 25 to 1000 μm, between highly polished 2 cm diam spherical electrodes, were extensively investigated at ∼10−6 Torr. The current–voltage characteristics of the above gaps obey Fowler–Nordheim behavior, confirming that the prebreakdown conduction was dominated by high field electron tunneling. The breakdown voltage derived from the prebreakdown data took the form of a power relation of gap distance as Vb=Kdα . Experimental results showed that highly polished metal electrodes can withstand very high fields in relatively poor vacuum: 220 V/μm for a 50 μm gap, 130 V/μm for a 300 μm gap. The breakdown of a narrow (⩽ 200 μm) gap resulted in rapid degradation, causing damage to both anode and cathode, while wider gaps (400–1000 μm) were found to exhibit spark conditioning, with the dc breakdown voltage increasing after each successive breakdown, causing damage only to the anode. A dc-glow-discharge treatment improved the gap insula...

Triple-junction issues in field emission displays

This article addresses triple-junction issues related to vacuum insulation in field emission displays (FEDs), both from a theoretical and practical point of view. A technique to inhibit the triple-junction initiated breakdown in dielectric spacer insulation has been demonstrated. In order to simulate the vacuum insulation structure in a FED, thin-film electrodes bridged by a thin-wall-structured spacer were used. Using the developed technique, the breakdown voltage was improved from ∼10 to ∼21 kV for the first run and ∼13 to 24 kV for the conditioned regime. The mechanism of breakdown of the solid insulator is discussed.

High field breakdown of narrow quasi uniform field gaps in vacuum

The challenge in vacuum microelectronic device design is to be able to stress a given micrometric gap to relatively high voltages without threat of a breakdown, which, in effect could destroy the device. In order to obtain basic vacuum insulation data related to the regime of vacuum microelectronics, the prebreakdown and breakdown characteristics of narrow gaps in the range of 3–25 μm were extensively investigated. The observed prebreakdown current was related to field emission from atomic scale microprotrusions or planar emission sites; the emission from these sites eventually produces breakdown. A single spark breakdown caused damage to both the anode and cathode. The dc glow discharge conditioning in air improved the insulation capability of narrow gaps (3–25 μm) significantly. The breakdown strength of a 5 μm gap after conditioning was as high as 5×108 V/m, which is the highest value reported in literature for broad area electrodes. It is shown that the electric field evaporation of metal ions from th...

High field breakdown characteristics of micrometric gaps in vacuum

In order to obtain the prebreakdown and breakdown data of plain vacuum gaps in the micrometric regime relevant to field emission displays, an automatic experimental system was developed. The dc breakdown characteristics of the gap in the micrometric regime, varying from 25 to 1000 μm, were extensively studied. The experimental results show that the highly finished broad-area metal electrodes withstand fairly high fields in relatively poor vacuum. The dc breakdown strength of a 50 μm gap was more than 220 V/μm at a pressure of 10−5 Torr, while for a 1000 μm gap, the breakdown strength was about 44.5 V/μm for highly polished chrome-steel electrodes, 2 cm in diameter. The breakdown of a narrow (<200 μm) vacuum gap resulted in rapid degradation, with the gap holding-off lower voltages after breakdown. However, a wider gap (500–1000 μm) was found to exhibit conditioning, with the dc breakdown voltage increasing after each successive breakdown. The breakdown characteristics of ideal gaps with very smooth solid electrodes, presented in this work, will define the theoretical limits to which actual gaps can be stressed in a field emission display.In order to obtain the prebreakdown and breakdown data of plain vacuum gaps in the micrometric regime relevant to field emission displays, an automatic experimental system was developed. The dc breakdown characteristics of the gap in the micrometric regime, varying from 25 to 1000 μm, were extensively studied. The experimental results show that the highly finished broad-area metal electrodes withstand fairly high fields in relatively poor vacuum. The dc breakdown strength of a 50 μm gap was more than 220 V/μm at a pressure of 10−5 Torr, while for a 1000 μm gap, the breakdown strength was about 44.5 V/μm for highly polished chrome-steel electrodes, 2 cm in diameter. The breakdown of a narrow (<200 μm) vacuum gap resulted in rapid degradation, with the gap holding-off lower voltages after breakdown. However, a wider gap (500–1000 μm) was found to exhibit conditioning, with the dc breakdown voltage increasing after each successive breakdown. The breakdown characteristics of ideal gaps with very smooth solid ...

High field performance of thin-wall spacers in a vacuum gap

This paper reports the electrical conduction and breakdown characteristics of thin-wall ceramic spacers for a field emission display (FED). These spacers bridge two thin-film electrodes, which represent the FED cathode and the phosphor anode in a FED. Techniques to set up a high aspect-ratio thin-wall spacer without glue were developed. An extra-low light detection 3D-imaging system using an intensified CCD camera was developed which was able to identify the location of low-level light activity in the stressed vacuum-gap, indicative of imminent device failure. Thin wall spacers made of various ceramics were investigated extensively. The scanning electron microscopy (SEM) surface investigation showed that zirconia spacers exhibited a smoother surface morphology compared to all other materials studied; however, their breakdown voltages were rather low. The breakdown voltages of alumina spacers were severely limited by triple junction effects. At HV, breakdown at the edge of the thin-film electrode was observed. This edge breakdown can be used to explain the saturation of the breakdown voltage vs. vacuum gap spacing. The results of this work are highly encouraging in that an /spl sim/1000 /spl mu/m tall spacer can support /spl sim/18 kV dc, at least 80% above the expected operational voltage of HV FED. The spacer breakdown voltage is expected to improve through surface treatment and elimination of the electrode edge-breakdown and triple junction effects.

A nondestructive automated defect detection system for silicon carbide wafers

Abstract.Manual methods of defect detection and classification on silicon carbide (SiC) wafers are tedious, time consuming, and prone to error. We have developed a nondestructive optical stress technique (photoelasticity) to isolate structural defects on SiC wafers. The technique is rapid, nondestructive, and inexpensive. In this paper we present an image processing system that exploits the optical system to detect structural defects on SiC wafers automatically. We are specifically interested in detecting defects known as micropipes. The philosophy of our approach is to reduce the dependency of the environment factors, image acquisition factors, and user parameters while maintaining the performance and computational speed. The goal is achieved by careful study of patterns that are invariant to the contrast and shift of pixel intensities and a combination of simple image processing techniques that are locally adaptive.

Prebreakdown and breakdown investigation of needle-plane vacuum gaps in the micron/submicron regime

This article reports investigations of insulation failure of needle-plane gaps of micrometric dimensions. Using a piezoelectric translational stage it is possible to set the gap spacing in the range between 0.1 and 40 μm with a resolution less than 0.04 μm. The prebreakdown and breakdown characteristics of the needle-plane gap were investigated as a function of the gap spacing, emitter radius, ambient pressure, and conditioning effects. The experimental results showed that the breakdown voltage of a microtip-plane gap is relatively high. For a 2 μm radius tip set at a 1 μm gap distance from a planar electrode, the breakdown voltage was about 500 V. It was found that an increase of pressure from 10−6 to 10−2 Torr did not influence the value of the threshold voltage for breakdown. dc glow discharge conditioning in air at a pressure of ∼1 Torr increased the value of threshold voltage for breakdown in vacuum by about 20%–30%. The obtained prebreakdown characteristics of microtip type gaps is very important fo...